A vertical alignment system, which utilizes a birefringent change of a liquid crystal layer when a nematic liquid crystal having a negative dielectric anisotropism is vertically aligned to a substrate and a voltage is applied by an electrode formed on the substrate, is known to show a high contrast as compared with a conventional TN system. As a liquid crystal alignment film to be used for such a vertical alignment system, a polyimide containing a long-chain alkyl group is disclosed, as described in e.g. JP-A-6-3678. However, with the liquid crystal alignment film to be used in such a vertical alignment system, it was difficult to carry out uniform alignment treatment by rubbing treatment as in the conventional TN system.
On the other hand, a vertical alignment system has been developed in recent years, wherein the direction of the electrical field is controlled by changing the structure of an electrode or projections formed on a substrate, whereby the alignment direction of liquid crystal is controlled during applying a voltage, without rubbing treatment. In such a vertical alignment system, a wide visual field angle property is obtainable as well as the high contrast, whereby a liquid crystal display device having a high display quality can be obtained. A liquid crystal alignment treating agent to be used in such a vertical alignment system, is usually coated on a substrate surface by means of a printing method and fired, whereby it is necessary to use a liquid crystal alignment treating agent showing high coating uniformity during the printing. Further, an active driving using TFT is employed for the liquid crystal display device of such a vertical alignment system, whereby it is necessary to use a liquid crystal alignment film having not only the vertical alignment properties, but also excellent electrical characteristics.
In the case of forming a liquid crystal alignment film, it is usually coated on a substrate by means of a printing method. However, a liquid crystal alignment treating agent for vertical alignment contains hydrophobic side chain substituents such as long-chain alkyl groups in order to align liquid crystal vertically, whereby there was a problem that when it was coated on a substrate, coating uniformity was not necessarily sufficient.
Further, in a case where a treating agent for vertical alignment is coated on a substrate and fired, it is usually fired at a relatively low temperature, since the heat resistance of the side chain structure is low as compared with the main chain structure. However, in a case where a polyamic acid is fired at a relatively low temperature of e.g. at most 200° C., there have been problems such as deterioration of the voltage retention properties due to an insufficient imidation reaction of a polyamic acid, or substantial deterioration of the display quality due to accumulation of a voltage in the liquid crystal display device during applying the voltage to the liquid crystal display device. Especially, it is important to have a high voltage retention property at a high temperature from the viewpoint of reliability of the liquid crystal display device. However, a liquid crystal alignment film for vertical alignment, which is prepared from the conventional polyamic acid, has not necessarily had a sufficient voltage retention property at a high temperature.
On the other hand, in a liquid crystal alignment film to be used for a liquid crystal display device in a TN method, a soluble polyimide as described in e.g. JP-A-2-287324, is known to have a high voltage retention property even if firing is carried out at a relatively low temperature. However, in the case of a vertical alignment film, the printing properties are substantially deteriorated by the effect of the side chain substituents as described above, whereby it is difficult to form a uniform coating film. Further, in the case of a soluble polyimide with a high content ratio of the hydrophobic side chain substituents, there was a problem that it did not have a voltage retention property as high as a liquid crystal alignment film to be used in a TN system.
Thus, the present invention is to provide a liquid crystal alignment treating agent for vertical alignment and a material for a liquid crystal alignment film for vertical alignment, which is excellent in voltage retention properties, particularly excellent in voltage retention properties at a high temperature, has a low accumulation voltage in a liquid crystal display device, and is excellent in film coating uniformity, even if it is a alignment film wherein the imidation reaction has not sufficiently proceeded, while it is excellent in printing properties, voltage retention properties and accumulation voltage properties in the same manner even if it is a soluble polyimide.